Using DirectoryOpus to copy files from a USB hard disk to my hard disk build-in my PC I get Access denied (error 5). Permissions are OK on both source and destination.
When I do the same file copy using File Explorer from WIndows 10 it works perfectly.
What is the problem?
Make a note of what's turned on under Preferences / File Operations / Copy Attributes, then turn it all off. Do you still get the access denied errors? (Set things back how they were after the test.)
If you do, which types of files are you copying?
If you're using thumbnnails mode, the viewer or metadata panes, or any file display columns that are generated by reading file contents, closing them may help (although you should not normally need to).
Are you literally copying the files, or moving them? Is UAC involved at all?